(1) Field of the Invention
The present invention relates to underwater acoustic sensing devices and, more particularly, to an expendable sonobuoy for forming virtual vertical sensing arrays.
(2) Description of the Prior Art
It is known to detect the location of an underwater object emitting sound, by utilization of a linear array of sound sensing devices, such as hydrophones. By recognition of the intensity of the sound, and the angle from which the sound is coming, there is computed a location for the sound source which, in military application, may be an underwater target, such as a submarine and in non-military applications may be a large animal, such as a whale, or a school of fish, or a sunken ship or other derelict. In general, the longer the array, the more accurate is the determination of the target location, particularly if the target is emitting a relatively low-frequency noise.
It is further known to extend the effective aperture (length) of an array of hydrophones by exploiting movement of the array during an observation period such that the effective aperture is equivalent to that of a longer, fully populated physical array having a length equal to the distance travelled by the moving array during the observation period. In U.S. Pat. No. 4,930,111, issued May 29, 1990, in the names of Edmund J. Sullivan and Stergios Stergiopoulos, there is disclosed a system utilizing a towed array of hydrophones, in which the effective length of the array is increased beyond the physical length of the array, to provide better definition of the target location.
In U.S. Pat. No. 5,040,157, issued Aug. 13, 1991, in the names of William I. Roderick and William A. Von Winkle, there is disclosed an expendable sonobuoy weighted to free-fall through the water, thereby forming a very long effective aperture, facilitating a long virtual vertical array. Acoustic field, temperature and depth information is transmitted from the sonobuoy to a host platform, such as a surface vessel, or an instrumented buoy which is in radio contact with an aircraft or surface vessel, via a communication line connecting the sonobuoy to the host platform. The sonobuoy transmits at preselected depth points along the effective aperture established by the free-fall of the sonobuoy to effectuate a very long virtual vertical array. The information signalled from the sonobuoy to the host platform is correlated to determine the range, depth and direction of the target. After the inputs from the virtual vertical array are accumulated, the communication line may be disconnected, the sonobuoy being thereby expended. The expendability of the sonobuoy is marginally acceptable in practice inasmuch as only a relatively inexpensive sonobuoy is required to generate a very large virtual vertical sensing array.
A difficulty which arises relative to use of the aforementioned sonobuoy is the fact that the sonobuoy is capable of only a single pass through the virtual vertical aperture. Given aberrations in the ocean underwater environment, low signal-to-noise ratio and the fact that targets may well be engaged in evasive maneuvers, it may be necessary in a given situation to expend more than a single sonobuoy, raising the cost, and increasing the required storage space for multiple sonobuoys on the host platform or other carrier.
Accordingly, it would be desirable to have available a sonobuoy having the sensing and signalling capabilities of the sonobuoy of U.S. Pat. No. 5,040,157, but, in addition, having the capability of generating a plurality of sequential virtual vertical arrays, such that one sonobuoy may fill the role of a plurality of the prior art devices.